Marshall attended the Juilliard School, where his mentor was John Houseman. Marshall's classmates included Kelsey Grammer, David Ogden Stiers, Kevin Kline and Robin Williams. Marshall first met Scott Bakula when performing in a production of Shenandoah. ("Marshall Decree", Dreamwatch issue 37)

Marshall made his first acting appearances in 1979, with appearances in the motion picture Tilt (with Don Stark) and a guest starring role in the How the West Was Won episode "The Forgotten", with Fionnula Flanagan and David Warner. In 1980, Marshall starred as Tony in a revival of West Side Story on Broadway.

Marshall later played the titular character in the 1982 series Marco Polo (with Leonard Nimoy, David Warner and F. Murray Abraham). He later starred in the films La Pelle, Claws and Krull, in which he played the main character Prince Colwyn. Marshall participated in the audio commentary for the DVD release of the film. His other motion picture credits include The Stay Awake, Feds (with Larry Cedar), The Gods Must Be Crazy II, Burndown, Berlin '39 (with John Savage) and Amberwaves.